handle modifications to this correctly (#3036)

fixes #3032

lib/compress.js

@@ -1052,6 +1052,7 @@ merge(Compressor.prototype, {
                         && (side_effects || node.expression.may_throw_on_access(compressor))
                     || node instanceof AST_SymbolRef
                         && (symbol_in_lvalues(node) || side_effects && may_modify(node))
+                    || node instanceof AST_This && symbol_in_lvalues(node)
                     || node instanceof AST_VarDef && node.value
                         && (node.name.name in lvalues || side_effects && may_modify(node.name))
                     || (sym = is_lhs(node.left, node))

test/compress/collapse_vars.js

@@ -5303,3 +5303,27 @@ issue_2974: {
     }
     expect_stdout: "1"
 }
+
+issue_3032: {
+    options = {
+        collapse_vars: true,
+        pure_getters: true,
+    }
+    input: {
+        console.log({
+            f: function() {
+                this.a = 42;
+                return [ this.a, !1 ];
+            }
+        }.f()[0]);
+    }
+    expect: {
+        console.log({
+            f: function() {
+                this.a = 42;
+                return [ this.a, !1 ];
+            }
+        }.f()[0]);
+    }
+    expect_stdout: "42"
+}